I thought the free space below the cp on a variety of cab designs was meant for more comfortable bar stool sitting? if stool's the plan, well...
Unless one plans to use all the space stuffing a ton of hardware inside that slim cab, I see no problem with shoving something heavy at the bottom of it.
It can be anything really, a bag of dirt or a dead animal, whatever, it's not like everyone's going to see it.
EDIT: Yes it's not an ideal solution, for effectiveness it need to be very heavy. The 20kg stone made things better but twice that would have been much better. Sure; the heavier the more room it takes...
When you try and create slim and compact designs though, why not go all the way and try to create the thing you're really dreaming about even if that means being inventive and making difficult compromises, tweaks/hacks?
Kinda like when building a custom SFF PC.
Custom shape also means custom weight distribution;
(ok it doesn't have to be that big, but you get the idea)